PROJECT SUMMARY/ABSTRACT Cerebral palsy (CP) results from a perinatal brain injury and is one of the most prevalent and costly pediatric neurologic conditions in the United States that often results in mobility deficits. Our extensive experimental work has been focused on developing a therapeutic gait training protocol that will improve the long-term mobility of adolescents with CP. Despite our major breakthroughs, it is apparent across our investigations that some adolescents have vast improvements in their mobility after treatment, while others are clearly non-responders. This response variability represents one of the major Gordian knots in the treatment outcomes seen in adolescents with CP. Over the past decade, we have begun to disentangle this knot through a series of influential magnetoencephalographic (MEG) brain imaging studies that have revealed that adolescents with CP have aberrant sensorimotor cortical oscillations while planning their leg motor actions and perceiving somatosensory feedback. Our preliminary work on the gait treatment outcomes and neuroimaging of the sensorimotor cortices has positioned us to be on the leading-edge for addressing RFA-HD-20-005, which requests ?studies to understand the underlying mechanisms for responders and non-responders to current treatments?. Essentially, we will address this call with a multimodal approach that blends our expertise in MEG brain imaging, MRI spinal cord imaging, and assessments of the spinal cord interneuronal circuitry to probe the neurophysiological differences in adolescents that are classified as responders and non-responders after therapeutic gait training. The Specific Aims of this study will: (1) identify if responders and non-responders have differences in the strength of the sensorimotor cortical oscillations involved in the planning and execution of a leg motor action, (2) determine if responders and non-responders have differences in the neural synchrony within the somatosensory cortices, following stimulation of the foot mechanoreceptors, and (3) decipher if responders and non-responders have differences in the spinal cord microstructure and circuitry dynamics. Briefly, our study design consists of a cohort of adolescents with CP that will initially undergo MEG brain imaging, MRI spinal cord imaging, neurophysiological tests of the spinal cord interneuronal circuitry and clinical mobility assessments. After completing the baseline tests, the adolescents will undergo our therapeutic gait training protocol. Upon completion of the treatment program, the adolescents will repeat the same assessments that were completed at baseline. Separation of the adolescents into responders and non-responders to treatment will be based on the criteria for a clinically relevant change in the 10-meter walking speed test. The new data derived from this project will provide unparalleled insight on the potential neurophysiological origins of the diverse mobility outcomes seen in adolescents with CP after treatment. These insights will set-the-stage for future studies that will explore the use of neuroscience-informed therapeutic approaches that are directed at breaking through the specific neuro- physiological barriers that non-responding adolescents may face (i.e., motor planning, sensory perception).